Strolling down the sidewalk at a park with a friend and chatting with them isn't necessarily a private activity either: We're in a very public space. Anyone within earshot can hear whatever we're talking about. If the sounds of our conversation winds up being incidentally in the background of someone filming the squirrels the tree frogs or something, then there's probably nothing to be done about that.

But (in some states), it seems that it would be a very different can of worms if I were to elect to deliberately record the conversation I have with my friend without their consent. Even in a public space, that would appear to run directly afoul of the applicable laws.